In fluid supply piping systems, it is known to use atmospheric-type vacuum breaker valves to prevent the backward siphoning of water or other liquid, which may be contaminated, from an outlet of the supply line to the supply source. The problem arises when a drop in pressure in the line on the supply side of the valve creates a partial vacuum in the supply line tending to draw liquid backwards into the supply line.
Vacuum breaker valves typically have a vent which is open to atmosphere when pressurized liquid is not supplied to the valve. When pressurized liquid is supplied to the valve, a valve piston moves to a vent closing position, enabling the liquid to be directed into the discharge piping. When internal pressure within the valve changes rapidly, such as during opening or closing of the valve, pressurized air and liquid within the valve can discharge through the vent opening while the valve piston is in an intermediate position causing undesirable and potentially harmful liquid leakage from the valve to the surrounding equipment.